Electrochemical cells based on lithium and other high energy materials are sensitive to how they are used. If the battery cell is mishandled it can lead to an internal short or external short, that can result in fire or sparks, that could further lead to an explosion. The problem is that most components used in association with an electrochemical cell are designed to protect the battery from such results ever occurring; there are few external measures that can be used to minimize the damage when fire or sparks do occur.
Electrochemical cells can have internal mechanisms, such as temperature activated separator paper, that shut the electrochemical cell down if over heated. As expressed by Darcy et al. in “Lithium-Ion Cell PTC Limitations and Solutions for High Voltage Battery Applications” (2003), many commercial, cylindrical lithium-ion battery cell designs are equipped with a positive thermal coefficient (PTC) current limiting switch to provide hazard protection against short circuits external to the cell. This PTC current limiting switch is a thin annulus consisting of a specially irradiated polyethylene laminated with a metal on both sides. When exposed to an overcurrent situation, this normally conductive polymer heats up and changes phases to become several orders of magnitude more resistive. Once the short is removed, the PTC cools down and returns to its electrically conductive state. This device has been a very effective method of providing reliable short circuit protection in low voltage battery assemblies.
However, when 8 or more fully charged cells in series are shorted, the first PTC that trips in the series strings can experience a large voltage drop that exceeds its voltage rating (˜30V) and will cause it to fail. Such tests performed at various research facilities have revealed that sparks and flames accompany those failures. The PTC usually fails shorted, becoming a charred substance. In a large series string, then the first cell PTC that fails shorted will transmit the problem to the next PTC to trip and the cascading series of flames and sparks will follow. This occurs because slight manufacturing variances in the resistance and trip points of the PTC prevent them from tripping in unison to distribute the large short circuit voltage drop among them.
A simple electrical protection scheme using bypass diodes is proposed to protect the battery cell PTCs from overvoltage conditions. The idea consists of placing a bypass diode in parallel with groups of 6 cells in series. During a short, the diode in parallel with the first PTC to trip will shunt the current away from the PTC until other cell PTCs in other series group of cells trip and share the voltage drop from the short circuit condition. This scheme protects the PTC from large voltage drop surges until the drop can be equitably distributed along the long series string of cells. That electrical solution, however, fails to address how to suppress flames and sparks when such deleterious events do occur.
Other cells have a thermal cutout (TCO) fuse device that is permanent and essentially operates like the above-identified PTC. The problem with a TCO device is that it protects against an electrical issue, and not against a mechanical issue such as a tab sticking into the can or an internal short being created. That electrical TCO solution, however, fails to address how to suppress flames and sparks when such deleterious events do occur.
Electrochemical cell vendors often use a vent or a weakened point on the battery cell's casing to controllably break open the casing when the electrochemical cell's internal pressure reaches a predetermined threshold that is too high for the cell's safe operation. The above-identified venting solution inhibits the cell from an uncontrolled electrical failure. That venting solution does not inhibit flames or sparks from shooting from the cell. Thus, the venting solution does nothing to minimize the potential flame or spark damage.
In U.S. Pat. No. 7,476,468; Lam discloses a flame retardant battery. The battery has a case and within the case is an electrode assembly. The electrode assembly has an electrolyte. Surrounding—from the right side, the left side, below, above or combinations thereof (col. 9, lines 22 to 51)—the electrode assembly and within the case is a fire suppressant material. Lam expresses the fire suppressant material is “primarily located outside” and is “substantially excluded from” the electrode assembly. Lam admits the fire suppressant material and electrolyte material may mix through surface tension and the curvature of a meniscus of the two liquids. Accordingly, there is no structural barrier between the fire retardant material and the electrolyte. Such potential interaction between the electrolyte and the fire suppressant material, when the battery cell is operating, is undesirable for the simple reason that the specific electrolyte may be adversely affected.
Tsukamoto, in US published application serial number 2009/0280400, discloses a battery pack contained in a case. Within the case is a porous medium that has openings to receive the battery pack. (See paragraphs 0018 to 0021) In addition to holding the battery pack, the porous medium can also be saturated with a flame retardant. (See paragraph 0029) Tsukamoto acknowledges this embodiment has disadvantages— “the liquid can conduct electrical energy between different locations in the battery pack . . . [thus] self-discharge can adversely affect the cycling performance of the battery pack . . . . However, electrically insulating the liquid from electrical components in the battery pack becomes more difficult when the liquid is outside the pores of the one or more porous medium.” (See paragraph 0030). Obviously in view of Tsukamoto's teachings, Tsukamoto teaches against a fire retardant contacting a battery cell but elects otherwise—having the flame retardant directly contact the battery cell—to inhibit potential flames and sparks.
Cieslak, in U.S. Pat. No. 5,002,843, discloses that Kevlar® brand aramid is a known “battery separator [that] physically separates the positive and negative terminals of a given battery”, “has superior resistance to most chemical reagents . . . , has an outstanding stability to heat, and retains as much as 95% of strength after exposure to temperatures of 500° F., . . . also exhibits good stability to temperatures above that level, KEVLAR [® brand aramid] is also a flame retardant material”, and “has superior mechanical strength, and better chemical resistance. Thus, KEVLAR [® brand aramid] possesses many advantages over other materials, especially in regard to safety concerns.” Cieslak's teachings disclose that Kevlar® brand aramid is an excellent separator component used between a electrochemical cell's positive and negative terminals, and is a known fire suppressant material.
A fire-extinguishing system for a home range is disclosed in the Freedman U.S. Pat. No. 3,209,837 (issued Oct. 5, 1965). Freedman discloses an “inexpensive heat resistant material such as metal foil having a powder material, such as baking soda, adapted to extinguish a fire, loosely retaining with the convolution of the roll, the roll being anchored to the hood structure at the outer end of the sheet so that when released, it unwinds in a downward direction to disperse a fog of the powder material over the area which the hood overlies and thereby extinguish the fire which may have ignited at such area.” (Col. 1, lines 24 to 32) The sheet material is maintained in a roll by a heat responsive element, like a fusible link. Upon melting of the fusible link, the sheet unrolls, thereby depositing the fire-extinguishing powder onto the home range. Such a fire-extinguishing system is applicable when there is a sufficient space positioned between the fire-extinguishing system and the burning object to allow the heat resistant material to unroll. A fire suppressant system for battery cells normally does not have such available space since battery cells are often used in locations having restrictive space.
Ellis; in U.S. Pat. No. 4,661,398; expressed that Kevlar, at least as early as 1987, was made with an adhesive. In particular, Ellis wrote in the abstract, “The coating may be used alone, or in the form of an impregnated sheet of woven or non-woven fabric made from fiberglass, carbon, aramid (“Kevlar”), quartz, polyester, nylon, or other natural or synthetic or inorganic fibers. The impregnated fabric adds tensile strength and flexural modulus to the laminate and may be used as the bonding agent (adhesive) alone or in combination with the currently used adhesives (e.g. phenol-formaldehyde, urea formaldehyde, resorcinol, melamine, melamine urea, urea, etc.).”
The above-identified information confirms that liquid flame retardants and Kevlar brand aramid are materials that have been used in association with electrochemical cells to provide flame retardant properties. In particular, some liquid flame retardants can physically surround and contact (a) an electrochemical cell's electrolyte and (b) the exterior and interior surface of an electrochemical cell's casing even when such contact provides undesirable electrochemical cell characteristics. In addition, Kevlar brand aramid with adhesive backing was known prior to filing this application. As evident from the above information, it is common practice with electrochemical cells to have the liquid flame retardant physically contact the battery cell components. Contrary to the conventional teaching, the inventors have determined otherwise to maintain the desirable electrochemical characteristics and simultaneously suppress any undesired fires or sparks.